1. Technical Field
This invention relates generally to improving the efficiency of radio frequency power amplification in a wireless communication device transmitter, and more particularly, to a power amplifier circuit for providing constant bias current over a wide temperature range.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two way voice and data communications in a handheld telephone-like communication handset. While the different modulation and transmission schemes each have advantages and disadvantages, one common factor is the need for highly efficient power amplification. As these communication devices become smaller and smaller, the functionality provided by these devices continues to increase. One major concern when developing these handheld communication devices is power consumption. As the devices become smaller and smaller, the need to manage the amount of power consumed and dissipated becomes more and more critical. High efficiency power amplification decreases the amount of power consumed, thus maximizing the life of the power source of the device.
Most wireless power amplifier applications require high efficiency, gain and linearity over a broad range of operating temperatures. Because these features are typically difficult to achieve in the power amplification circuitry, additional circuitry and logic are required to achieve these goals.
Bias circuitry, that is typically present in power amplification circuits, is used to provide a xe2x80x9creference currentxe2x80x9d to the radio frequency (RF) power amplification device. A reference device, that can be for example but not limited to, a reference amplifier implemented using one or more transistors, provides the reference current. Th reference current is multiplied to generate a proper bias current for the RF amplification device. Unfortunately, the reference current fluctuates with temperature, resulting in undesirable performance variations in the RF power amplification device due to variations in the reference current caused by the temperature fluctuations.
FIG. 1 is a schematic view illustrating a conventional bias circuit for a radio frequency (RF) power amplifier. Power amplifier circuit 100 includes reference transistor 106 (designated xe2x80x9cQREFxe2x80x9d) through which a reference current flows. The reference current flows from the voltage source VREF through resistor 102 into the collector terminal 104 of reference transistor 106. The reference current then flows through the emitter terminal 108 to ground. The reference current also flows through connection 112 and through connection 110 to buffer circuitry 114. Connection 112 connects the base terminal 110 of the reference transistor 106 to the collector terminal 104 of reference transistor 106. The reference current then flows through buffer circuitry 114 to the base terminal 116 of power transistor 122. This reference current, sometimes referred to as the bias current, present at the base terminal 116 of power transistor 122 regulates the flow of current from the node VCC (RF OUT)through power transistor 122.
In operation, an RF input signal is applied to node 118 labeled xe2x80x9cRF INxe2x80x9d through capacitor 120 to the base terminal 116 of power transistor 122. The RF signal is then amplified by power transistor 122 as a function of the current flowing through the power transistor 122. The buffer circuitry 114, the detail of which has been omitted for simplicity, includes additional circuitry that allows the low beta transistors 106 and 122, each of which have high base currents, to support those high base currents. In this manner, the buffer circuitry 114 supplies the base current required by the power transistor 122. In addition, the buffer circuitry 114 includes circuitry that prevents the RF signal applied to node 118 from coupling back into the bias circuitry 130, and in particular, from adversely affecting the reference transistor 106. The output of the power transistor 122 is taken from the collector 124 at the node labeled xe2x80x9cRF OUT.xe2x80x9d The current flowing in the collector 124 is defined as IRN=*IREF, where N is a multiplication factor, or ratio, between reference transistor 106 and the power transistor 122.
The reference current flowing through the bias circuitry 130 may vary with temperature, resulting in an undesirable performance variation over temperature. For example, the bias current (reference current) includes temperature dependencies that follow the following equations:
Icqxe2x88x9dTxcex4/2xe2x80x83xe2x80x83Equation 1
Icqxe2x88x9dTxe2x80x83xe2x80x83Equation 2
where I is the reference current, cq is the quiescent current at the collector of the reference transistor, xe2x88x9d indicates proportionality, and T is the temperature.
Therefore, there is a need in the industry for a temperature independent wireless power amplification bias circuit that allows the power amplifier to achieve highly efficient power amplification over a broad range of temperature variations and that is economical to produce in high volume.
The invention provides a temperature stable bias circuit for an RF power amplifier. The bias circuitry uses current deletion and current supplement techniques to maintain the bias or reference current of the RF power amplifier at a stable level regardless of the temperature under which the power amplifier is operating. When temperature increases, the current deletion circuitry reduces the bias current supplied to the power transistor. When the temperature decreases, the current supplement circuitry increases the bias current supplied to the power transistor. The bias circuitry allows the output of the RF power amplifier to remain constant. The current deletion and current supplement circuitry can be fabricated using the same processing technology as the power amplifier and can be easily integrated into the power amplifier device packaging.
Related methods of operation and computer readable media are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.